This invention relates to an induction system for supplying air-fuel mixture to a two-stroke cycle, crankcase compression internal combustion engine, and more particularly to such a system having a single carburetor barrel for each cylinder.
In large, two-stroke cycle crankcase compression engines, it has been found that a satisfactory induction system is provided by a reed valve system mounted in the induction passage. The reed valve system includes a valve seat member having a plurality of valve ports arranged in a generally circular pattern equidistant from the axis of the induction passage, and a plurality of reed valve members, each normally closing one of the valve ports and opening in response to low pressure within the crankcase to admit air-fuel mixture thereto. Such a system is disclosed in U.S. Pat. No. 4,408,579, the disclosure of which is hereby incorporated by reference. A flow modifying body is mounted in the induction passage immediately upstream from the reed valve system, to provide efficient flow to the crankcase while preventing spit back through the carburetor. With the reed valve system mounted to the engine crankcase, this type of induction system provides a shorter length than prior art systems employing a V-type reed block.
The induction system as disclosed in U.S. Pat. No. 4,408,579 provides a highly satisfactory structure and operation for a large two-stroke cycle engine, e.g. 70 to 115 horsepower. On such large engines, the induction system as disclosed in U.S. Pat. No. 4,408,579 is readily adapted to fit the center-to-center spacing of the cylinders, which is relatively large due to the large displacement of the engine.
The present invention addresses an induction system for a smaller two-stroke cycle engine, such as one providing 30 to 60 horsepower. In a smaller engine such as this, the center-to-center spacing of the cylinders is much closer than in the larger engines as described above. It has been found that the system as disclosed in U.S. Pat. No. 4,408,579 requires more space than is available on a smaller engine. However, the principle and operation of the induction system structure as disclosed in U.S. Pat. No. 4,408,579 have been found highly satisfactory. The present invention addresses the problem of applying the theory of the induction system as shown in U.S. Pat. No. 4,408,579 to a smaller horsepower engine.
In accordance with the invention, a two-stroke cycle crankcase compression engine includes a crankshaft, a crankcase surrounding the crankshaft, an induction passage for supplying air-fuel mixture to the crankcase, and reed valve means mounted in the induction passage. The induction passage is preferably formed in an induction manifold casting disposed upstream of the reed valve means. The reed valve means includes a valve seat member, preferably a reed block, having a plurality of valve ports. The reed block is a one-piece member, and valve ports for each induction passage are formed in the reed block. The valve ports are arranged in a plurality of oppositely oriented sets. Preferably, two sets of three valve ports are provided for each induction passage. The valve ports in one set are disposed radially opposite the valve ports in the other set, and all of the valve ports are preferably equidistant from a center point offset from the axis of the induction passage. A plurality of reed valve members are provided for each induction passage, each normally closing one of the valve ports and opening in response to low pressure within the crankcase to admit air-fuel mixture thereto. The plurality of reed valve members preferably comprise a series of petals formed on a single reed valve, with the petals being arranged so as to cover the valve ports for one of the induction passages. A flow modifying body is preferably mounted in the induction passage immediately upstream from the reed valve means, and a carburetor having a barrel is mounted upstream of the flow modifying body, with the barrel having an axis substantially aligned with the axis of the induction passage.